1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to a speed control arrangement for the hydraulic actuator assembly of a push-type die closing unit of an injection molding machine which has auxiliary cylinders for rapid-travel opening and closing movements.
2. Description of the Prior Art
In my German Pat. No. 30 44 137 and my corresponding U.S. Pat. No. 4,380,427 is disclosed a hydraulic actuator assembly for a push-type die closing unit for injection molding machines which produces rapid die opening and closing movements with a small volume of pressurized fluid and creates an elevated die closing pressure against the closed injection molding die.
This actuator assembly consists essentially of a central power cylinder with a selectively bypassable power piston, two opening travel cylinders arranged alongside the power cylinder, on diametrically opposite sides of the latter, and a stationary closing travel plunger arranged inside the power cylinder piston rod.
The power piston divides the bore of the power cylinder into a low-pressure space on its forward side and a larger high-pressure space on its rearward side. The low-pressure space of the power cylinder communicates with the pressure spaces of the two opening travel cylinders, forward of their pistons, and the total effective area of the three connected pressure spaces is larger, by a relatively small amount, than the area of the high-pressure space of the power cylinder. Because this difference determines the fluid volume which is taken in during the opening movement, a rapid opening movement is obtained with a small amount of pressurized fluid.
The stationary plunger is arranged in a central bore in the rear portion of the power piston rod. With its relatively small pressure space, it produces a similarly rapid closing movement with a small amount of pressurized fluid. An elevated die closing pressure is obtained by closing the power piston bypass channels and by pressurizing both the high-pressure space to the rear of the power piston and the plunger pressure space.
This arrangement, while making it possible to obtain rapid opening and closing movements of the die closing unit with a small amount of pressurized fluid, may develop certain control problems, especially when a heavy die closing unit is operated at high speeds. The moving mass, consisting of the movable die carrier frame with its movable die half and the attached power piston and travel pistons with their piston rods, receiving its force input from a comparatively small pressure space during the opening and closing movements, may develop undesirable movement fluctuations under deceleration at the end of the movement strokes.
The compactness of this hydraulic actuator assembly makes it possible to tilt the die closing unit into a vertical orientation, for injection into the partition plane of the injection molding die. However, additional control problems can arise in this orientation, due to the fact that the weight of the moving mass adds to, or subtracts from, the forces which are required for acceleration and deceleration of this mass.